The present invention relates to the field of semiconductor devices and their manufacture. More specifically, in one embodiment the invention provides both bipolar and complementary metal-oxide semiconductor (CMOS) devices on a single substrate and a process for their fabrication.
Bipolar and CMOS devices and their fabrication have been well known for many years. Recently, the advantages of both types of devices have been beneficially incorporated into circuits using both types of devices on a single substrate. Circuits which incorporate both bipolar and CMOS devices have come to be known as "BiCMOS." BiCMOS devices offer the advantages of the high packing density and low power consumption of CMOS devices, as well as the high speed of bipolar devices. One BiCMOS device and process for fabrication thereof is described in U.S. Pat. No. 4,764,480 (Vora), assigned to the assignee of the present invention and incorporated herein by reference for all purposes.
While meeting with some success, BiCMOS devices continue to have certain limitations. For example, oxide encroachment in the isolation process reduces the packing density of the CMOS structures. Further, the isolation processes used in prior devices prevents close spacing of the transistors due to the associated reduction in latchup immunity. Still further, it has been necessary to provide a punchthrough implant in scaled MOS devices to prevent punchthrough. Still further, source/drain resistance and source/drain contact resistance has limited the current drive capability of the MOS transistors and the packing density of the CMOS structures has been limited by direct metal contacting of the source/drain regions. Also, capacitance of the source/drain junction limits the AC performance of the CMOS structures.
Extrinsic base resistance is in some instances an important factor in AC performance of bipolar structures. Still further, collector-substrate junction capacitance degrades the speed of bipolar circuits.
From the above it is seen that an improved BiCMOS device and method of fabrication thereof is desired not only to provide devices with improved performance and reduced size, but also to provide devices which can be fabricated more simply and economically.